He spoke so fast that I could not follow.
It was so easy a question that every one of us could answer it.
Much to his surprise,none of them could tell the difference between them.
这类个例句中的could是表示过去的能力吗?她们能否用 was able to来替代?
1. He spoke so fast that I could not follow.
2. It was so easy a question that every one of us could answer it.
3. Much to his surprise, none of them could tell the difference between them.
先说答案：这三个句子 could 都可以换为 was/were able to，由于就“能力” 而言，后者的意思包括着前者的意思。
could 只表示有这种“能力”，不表示“能力的结果”。was/were able to 既能够表示“能力”，还可以表示“结果”，即：它包括着 could 的意义。
了解了这个不同，那样，在一些不一样的句子中，需要依据语义选择不一样的表达。比如：
He didn’t agree with me at first, but I ________ persuade him to sign the agreement later.
A. could （误）B. was able to （正）
could 表示“有能力做某事”，was able to 除去这层意思外，还有一个要紧的意思，就是：“设法做了某事”，“成功地做了某事 ”。假如只表达“有能力做某事”这层意思，那样这两个可以互换，而意思维持不变，比如：
He was very strong and he could lift that big stone. 他非常强壮，他可以举起那块大石头。
但，假如有能力且做到了，那样，只可以用 was able to，不可以用 could，比如：
At last he was able to solve the problem.（正）
At last he could solve the problem.（误）
only when the war was over was he able to get happily back to work.（正）
only when the war was over could he get happily back to work.（误）
但，当句子是不是定意思时，could 和was able to 没差别：
I couldn’t persuade him to sign the agreement. = I wasn’t able to persuade him to sign the agreement.